Question Video: Identifying Types of Orbit Physics • 9th Grade

The diagram shows two different possible orbits of an object around a star. Which of the following correctly describes the shape of orbit (a)? [A] Elliptical [B] Circular [C] Helical [D] Spiral [E] Highly elliptical


Video Transcript

The diagram shows two different possible orbits of an object around a star. Which of the following correctly describes the shape of orbit a)? A) Elliptical, B) Circular, C) Helical, D) Spiral, E) Highly elliptical.

All right, taking a look at our diagram, we see these two orbits marked a) and b). And this question focuses on orbit a). We want to know which of these five paths correctly describes this particular orbit. Now, the first thing we can do is recall that whenever one object is in orbit around another one, there are two possible paths that that orbiting object can follow. Orbital paths are either circular or they’re elliptical. These are the two allowed shapes we could say for orbital motion. That eliminates two of our possible answer choices, option C and D. Helical and spiral are not allowed orbital paths.

At this point, we can recall the difference between these two allowed orbits of circles and ellipses. We all know what a circle looks like. And an ellipse looks like a circle that’s been squished or compressed. So if this was a circular orbital path, an elliptical path would look something like this. Now, we can see that our three remaining answer choices are elliptical, circular, and something called highly elliptical. For an orbit to be highly elliptical, we could think of that as a very compressed circular orbit. The squishing, we could say, has gone even farther to create something that looks like this. This would be an example of a highly elliptical orbit. So which of these three — elliptical, circular, or highly elliptical — is the orbit marked out in a)?

We can see that this orbit does not look like a compressed circle, but rather it just looks like a circle itself. It’s definitely not highly elliptical then. We’ll cross off option E. And we also don’t think it’s elliptical. The shape of orbit a) is circular. Now, let’s consider the same question, but about orbit b).

Our next question asks, which of the following correctly describes the shape of orbit b)? A) Circular, B) Spherical, C) Highly elliptical, D) Spiral, E) Helical.

Just like before, we can cancel out any answer options which are neither circular nor elliptical. As we’ve seen, those were the only two possible paths that an orbiting body can have. So this means that option B, spherical, option D, spiral, and option E, helical, are off the table. So then, orbit b) is either circular or it’s highly elliptical. If we revisit our sketch of a circular orbit, an elliptical orbit, and a highly elliptical orbit marked out in yellow, we can clearly see that orbit 𝑏 is not circular. This eliminates option A. And as we consider highly elliptical as a description of this shape, we see that there’s a match. This orbit has a very compressed or squished look compared to a circular orbit. And, therefore, it is highly elliptical. This is the correct description of the shape of orbit b).

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